456 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



If the latter is a shale, the first bed of the succeeding formation is likely 

 to be an amorphous or obscurely stratified clay shale. If it is a lime- 

 stone, then the new bed commonly is a highly calcareous, thin, and irreg- 

 ularly distributed mudrock. The resemblance is greatest when the sea 

 transgresses over a sandstone surface. In such cases, as, for instance, 

 in Arkansas and Missouri, where the Sylamore or some even younger 

 sandstone is occasionally found in contact with the St. Peter or some 

 other sandstone, it may be very difficult to draw the line between the old 

 and the reworked new deposit. 



Theoretically the littoral deposits of a sea should be of coarser grain 

 than those laid down at depths beyond the reach of wave action. Strictly 

 speaking, the facts are in essential accord with the theory, but viewed 

 from the side of practical stratigraphy the difference is rather generally 

 of small consequence and often quite negligible. Although a large 

 amount of data respecting near-shore deposition in Paleozoic continental 

 seas is in hand, the space here available permits only the briefest refer- 

 ence to a few. However, fuller references to these and other cases of 

 obscurely indicated littoral deposits will be found in this and preceding 

 chapters. 



LITTORAL CONDITIONS RECORDED BY INITIAL DEPOSITS 



Accepting as an established fact that nearly all diastrophically deter- 

 mined formations have an overlapping structure — the exceptions being 

 formations based solely on lithologic differences — it follows that the basal 

 layers of each constitute a continuous local record of littoral conditions 

 during the time consumed in the respective transgressions of the sea. 

 In each exposure of such basal layers we see, therefore, a representation 

 of the littoral deposits of the sea at a particular time and place. Com- 

 parative studies of stratigraphic units carried on under this conception 

 show that in the case of limestone formations the littoral deposits are but 

 seldom, and at that only locally, conspicuously more arenaceous than the 

 deeper water sediments. The relatively common exceptions are lime- 

 stone formations, like the Boone in northern Arkansas, which trans- 

 gressed old land surfaces, and in such situations begin with a few inches 

 of sandstone. Despite the prevailing contrary conception, unquestion- 

 able major exceptions are few indeed. The best that comes to mind at 

 this moment is the Joachim, a magnesian limestone in Arkansas, Mis- 

 souri, and western Illinois that is regarded as having been deposited in 

 the wake of the northwardly advancing St. Peter sea. 



In a great majority of the limestone cases there is no sign of quartz 

 sand at the base ; and when such material is present it is usually confined 



